Method of making a polyethylene terephthalate laminate

ABSTRACT

A container is provided formed from a blank made of a laminate which includes nonbrowning paperboard and at least an inner layer of substantially amorphous polyethylene terephthalate bonded to the paperboard by its own substance. A method of making this laminate is provided including the steps of heating an uncoated surface or both surfaces of the paperboard to a temperature of at least about 285°F. and extruding the polyethylene terephthalate layer onto this surface or these surfaces. A method of cooking is provided including the steps of forming a container from the blank, placing food in the container and cooking the food at a temperature of up to about 400°F.

This is a division of application Ser. No. 281,943, filed Aug. 18, 1972,U.S. Pat. No. 3,924,013.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is a cooking method using a novel container formed from ablank of a novel material (i.e., a laminate of at least a layer ofpolyethylene terephthalate and nonbrowning paperboard). This inventionis also a method of making this novel laminate including the step ofheating, at critical temperatures, an uncoated surface of surfaces ofthe paperboard to improve adhesion between it and the polyethyleneterephthalate material extruded thereon.

2. Description of the Prior Art

The following prior art is of interest to the laminate and its method ofmanufacture of this invention:

U.S. Pat. No. 3,402,086 to Smith et al. shows a process for hot-meltextrusion coating of paperboard or paper with a randon copolymer ofethylene containing α,β-ethylenically unsaturated monocarboxylic acidwherein the paper is preheated by hot rolls to a temperature within therange of 80°C. to 250°C. prior to being melt coated.

U.S. Pat. No. 3,498,865 to Psquin et al. shows a continuous process forcoating paper with a film of extrudable plastic, such as polyethylene,wherein the air in the region of convergence of paper and plastic filmis ionized. The specification discloses that the paper which contactsthe molten polyethylene is heated by use of steam-heated rolls to atemperature of 65°C. to 115°C. Infrared or other heating elements mayalso be used for heating the paper.

Canadian Pat. No. 529,361 to Fields shows a process for extrusioncoating a synthetic linear polymer having a sharp melting point onto asheet. Polyesters are disclosed as useable materials for the coating andpaper is included as a material that can be used for the sheet.

Canadian Pat. No. 781,424 to Marshall discloses a method of coating aporous web with a continuous film of a thermoplastic polymer wherein theporous web is preheated before the application of the molten film of thepolymeric material. Porous paper may be used as the porous web materialand polyesters may be used for coating the porous webs.

British Pat. No. 1,119,804 to Rohm & Haas Company discloses a laminatecomprising a substrate such as a sheet of paper and a layer ofpolyhexamethylene terephthalate which has been melt-extruded thereon.

A prior art patent of interest to the container and cooking method ofthis invention is U.S. Pat. No. 3,276,900 to Funck, which shows the useof a paperboard component in a container used in cooking foods atrelatively high temperatures.

Another patent of interest in the food container art is U.S. Pat. No.3,316,102 to Doll et al. which shows a food tray blank, container andpackage, and which blank includes means for locking parts of thecontainer together to hold it in erected position without use ofadhesives or mechanical fasteners.

U.S. Pat. No. 3,550,835 to Persson is also exemplary of the containerand cooking arts. Such patent teaches a package especially for foodproducts, which package is a tray erected from a paperboard blank whichhas at least on one side a fluidtight plastic coating, i.e.,polypropylene, which is adapted to be sealed and withstands heating toabout 305°F. While this package is suitable for relatively low cookingtemperature use, it cannot withstand heating at the high temperaturesabove 325°F. and to about 400°F., which temperatures are those mostcommonly used in the precooked, convenience food areas of cooking.

SUMMARY OF THE INVENTION

According to the present invention, a container is provided for use,preferably, in the packaging of food and for use in the subsequentcooking or reheating of the food in preparation for human consumption.The container is formed from a blank made of a material comprisingnonbrowning paperboard with polyethylene terephthalate material extrudedonto one or both of its surfaces. The method of making such material inwhich an uncoated surface or surfaces of the paperboard is heated,preferably by flame, enables a bond to be set up between the paperboardand polyethylene terephthalate that is effective in use applications ofthis sort. In forming the container from the blank of this invention,the polyethylene terephthalate surface, or parts of it, may be broughtinto abutting contact with other parts of the polyethylene terephthalatesurface or with parts of the paperboard surface and secured orheat-sealed together, or the container may be formed by appropriatelyfolding the blank or by heating the blank and die pressing it intocontainer form. Food is then placed in the container, which food maythen be reheated or heated to temperatures up to about 400°F. withouthaving browning problems with the paperboard and without havingdelamination problems with the laminate.

This container does not deleteriously effect the cooking of the foodcontained therein and may be formed at low heatsealing temperatures andcooked or used at high cooking temperatures and still remain attractivein appearance.

Cooking containers are well known to the art. Many types of materialsare used to frabricate these containers. Such materials can generally becharacterized into one of three primary types or combinations of them.These types are paperboard, aluminum and certain thermoplasticmaterials. There are advantages and disadvantages resulting from the useof each of these types of materials in fabricating food containers. Forexample, the nonpaperboard containers are generally nonbiodegradable andthe aluminum containers are difficult to hermetically seal and cannot beused in microwave ovens; paperboard containers, on the other hand, donot have these disadvantages.

Despite the widespread use of paperboard in the packaging industry,however, certain limitations in its utility have long been recognized.For example, uncoated paperboard packages are weakened by moisture andare not impervious to greases and oils and char and brown at highcooking temperatures. Further, this paperboard browning is still animportant, unsolved, problem factor even when the paperboard is coatedwith certain plastic materials; i.e., polypropylene coated paperboard"browns" when heated to temperatures of about 305°F. Consequently, whenpackaging certain types of foods for cooking, paperboard may lack a keyproperty relating to the appearance of the package. Other physicalcharacteristics of paperboard have limited the forms of the package andthe types of surface treatment which could be used on paperboardpackaging materials.

Convenience foods, for example, may be precooked and packaged in certaincontainers and then reheated. For this reason they must be capable ofwithstanding this subsequent food-heating operation without causingadverse effects with the food or the package. The containers used forthis should be capable of being subjected to temperatures of up to400°F. Prior to this invention, known paperboard containers could not beso employed primarily because such containers easily char when subjectedto temperatures of this order and the browning destroys the aestheticsof the container.

The instant invention provides novel containers formed from a blank madein part of paperboard which exhibits unique packaging versatility andwhich overcomes disadvantages, such as browning at high temperatures, ofcurrently used container materials. In its broadest terms, thisinvention is a disposable container which comprises a layer ofnonbrowning paperboard and at least a layer of substantially amorphouspolyethylene terephthalate bonded to the paperboard layer by its ownsubstance. This includes, for example, containers formed from blanks byfolding, die-pressed containers and the like.

Specifically, the instant invention in one embodiment provides acontainer for baking and cooking which comprises:

a substantially flat bottom part;

at least four upstanding sides hingedly connected to the flat bottom;

corners joining adjacent upstanding sides hingedly connected to the flatbottom parts;

the container being formed of a 2-ply laminate comprising a layer ofnonbrowning paperboard and at least a layer of substantially amorphouspolyethylene terephthalate bonded to the paperboard layer by its ownsubstance; and

the corners being formed by heat-sealing surface portions of thesubstantially amorphous polyethylene terephthalate layer of adjacentsides together and the polyethylene terephthalate in the corners beingpartially crystalline, thereafter.

In another embodiment, this invention is a container for baking andcooking which comprises:

a substantially flat bottom part and body parts connected thereto;

the container being formed of a laminate comprising a layer ofnonbrowning paperboard and at least a layer of substantially amorphouspolyethylene terephthalate bonded to the paperboard layer by its ownsubstance; and

the container having abutting surface portions of the body parts securedtogether.

In this container at least one and, if desired, both, of each of theabutting surface portions of the body parts is of the polyethyleneterephthalate layer and such abutting surface portions may beheat-sealed together or secured together by other means, such as staplesor adhesives, or by interlocking parts, for example.

All of these containers may be formed from a container blank of thisinvention having parts including a bottom part with body parts connectedthereto, such blank being formed of a laminate including a nonbrowningpaperboard having a least an extruded layer of polyethyleneterephthalate bonded thereto.

A method of forming a container from this blank includes the steps of:folding the blank whereby surface portions of body parts are broughtinto abutting contact with each other, all of such surface portionsbeing of polyethylene terephthalate; and heat-sealing such abuttingsurface portions together to form a container. Such abutting surfaceportions are heat-sealed together at a temperature of at least about320° F.

Another method of forming a container from the blank includes the stepsof: folding the blank whereby surface portions of the body parts arebrought into abutting contact with each other; and securing suchabutting surface portions together to form a container. Such abuttingsurface portions may be heat-sealed together at a temperature of atleasst about 320° F. or secured together by other means. At least one ofeach of such abutting surface portions is of the polyethyleneterephthalate layer and the other of each of such abutting surfaceportions may be of the nonbrowning paperboard.

Still another method of forming a container from the blank includes thesteps of: heating the blank to a temperature of at least 320°F., andpressing the blank to form a container.

And, further, a method of this invention is provided of making alaminate suitable for use in making these blanks and containerscomprising the steps of which include: passing a nonbrowning paperboardweb having a pH of between 7.0 and 7.5 adjacent a heating source;heating an uncoated first surface of the paperboard to a temperature ofat least about 285°F. by means of the heating source; extruding a layerof polyethylene terephthalate onto the uncoated first surface of theheated paperboard, such polyethylene terephthalate having an intrinsicviscosity within the range of 0.51 to 0.85; and cooling the extrudedlayer of polyethylene terephthalate to solidity it and to effect a bondbetween the paperboard and the polyethylene terephthalate to form a2-ply laminate.

A method of this invention further is provided of making a 3-plylaminate suitable for these same uses wherein the other surface of thepaperboard of the 2-ply laminate also is uncoated and wherein thepaperboard is heated to a temperature of from about 285°F. to about340°F. including the additional steps of: extruding a layer ofpolyethylene terephthalate onto such uncoated second surface of theheated paperboard, such polyethylene terephthalate having an intrinsicviscosity within the range of 0.51 to 0.85; and cooling this extrudedlayer of polyethylene terephthalate to solidify it and to effect a bondbetween it and the paperboard to form the 3-ply laminate.

And, lastly, a method is provided for cooking in these novel containersformed from blanks made of these novel 2-ply and 3-ply laminates, whichcooking method includes the steps of: forming a container from a blankformed from a 2-ply laminate consisting of a nonbrowning paperboardlayer having a layer of polyethylene terephthalate extruded thereon andbonded thereto by its own substance by the methods previously described;placing food in the container; and, cooking the food at a temperature ofup to about 400°F.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagrammatic view of apparatus suitable for making a 2-plyand 3-ply laminate of this invention;

FIG. 2 is a fragmentary plan view of a corner blank made from a laminateof this invention (with parts omitted for clarity); and

FIG. 3 is a perspective view showing a container formed from the blankof FIG. 2.

FIGS. 4 and 5 are perspective views of another container of thisinvention, showing a locking tab body part before and after it is tuckedinto position to embrace and retain folded side extension body parts inplace.

FIG. 6 is a perspective view showing still another container of thisinvention (i.e., a cooking plate) which may be formed by die pressing asuitable blank made from a laminate also of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, and in particular FIG. 3, there is shown atypical novel container C of this invention.

Such container C is formed from a blank B, of the type shown in FIG. 2,which blank is suitably cut and scored to define parts including abottom part 10 with body parts or sides 11 hingedly connected theretoalong fold or score lines 12. Other fold lines, such as those designated13, extend from the fold lines 12 and define means for enabling theblank to be folded at the corners 14 thereof in forming the container C,in a manner to be described.

In forming the container C from the blank B the sides 11 are foldedupwardly with respect to the bottom part 10 and corner portions 14' ofthe inner surface of adjacent sides 11 are brought into abutting contactwith each other and secured (i.e., heat-sealed) together to form thecorners 14 of the container C from the corner portions 14. The materialof the inner surface of the blank B is such that it is heat-sealable toitself or to paperboard layer 15 as will be further explained.

The blank B is made from a novel laminate of this invention consistingof a layer of nonbrowning paperboard 15, such as so-called "14 pointboard", having a layer of polyethylene terephthalate 16 extruded onto anuncoated first surface or such paperboard 15 in a novel manner andbonded to such paperboard by its own substance to form a 2-ply laminate17, as best shown in FIGS. 1 and 3. If desired, another layer ofpolyethylene terephthalate 16' may be extruded onto the other uncoatedsecond surface in accordance with this invention to form a 3-plylaminate 17'.

Other typical containers C' and C" formed from the 2-ply and 3-plylaminates 17 and 17' of the invention are shown in FIGS. 4-6.

The container C' is formed from a suitably cut and scored blank, asshown, for example, in U.S. Pat. No. 2,373,730, which blank includes abottom part 10' and body parts 11' connected thereto by folding theblank whereby surface portions of the body parts 11' are brought intocontact with each other and secured together (i.e., heat-sealed orinterlocked, etc.) to form the container. In other words, in forming thecontainer C', the locking tab body part 18' is tucked into the positionshown in FIG. 5 to embrace and retain the folded side extension bodyparts 19' in place, which tucking secures the surface portions of thebody parts together.

It is noted that in this position surface portions of the body parts arebrought into abutting contact with each other with parts of thepolyethylene terephthalate layer 16 surface in contact with parts of thepaperboard layer 15 surface. While the tucking of the tab 18' intoposition above will secure the surface parts together, if desired, theparts may also or only be heat-sealed together to do the same. Thisinvention, then, includes the heat-sealing of surface portions of thepolyethylene terephthalate layer to other portions of that layer as wellas to the paperboard layer and further includes the interlocking ofthese parts together, in forming a container of this invention.

The container C", as shown in FIG. 6, is still another example of acontainer formed using a laminate 17 or 17' of this invention, whichcontainer (i.e., a cooking plate) may be formed from a suitably shapedblank by heating the blank to at least 320°F., and by pressing theheated blank to form a bottom 10" with body portion 11" connctedthereto.

The specific type of paperboard 15 used in forming a laminate of thisinvention is critical. Among its most important characteristics is thatthe surface of such paperboard to which the polyethylene terephthalatelayer is to be fused preferably is uncoated and, if coated at all,should not be clay coated. It has been found that unless this is so theadhesion properties are less than those desired.

Other properties of the paperboard which are required when it iscombined with a polyethylene terephthalate layer for use in cooking athigh temperatures include the property of nonbrowning. It has been foundto be critical that the pH must be controlled between 7.0 and 7.5 toprovide a paperboard that does not brown at cooking temperatures of upto about 400°F.

As to nonbrowning, it has been found that the surface of the paperboardopposite that to which the polyethylene terephthalate layer is adheredmay be coated with a clay coating, if desired, and that it is stillpossible to obtain excellent adhesion to the noncoated side of thispaperboard.

The polyethylene terephthalate material of this invention is formed froma polymer formed by the condensation reaction of ethyl glycol andterephthalic acid. Particulars of this material are shown and describedin U.S. Pat. No. 2,465,319 to Whinfield and Dickson. More specifically,polyethylene terephthalate useful in preparing the laminate of thisinvention includes (a) polymers wherein at least about 97% of thepolymer contains the repeating ethylene terephthalate units of theformula: ##EQU1## with the remainder being minor amounts ofester-forming components, and (b) copolymers of ethylene terephthalatewherein up to about 10 mole percent of the copolymer is prepared fromthe monomer units of diethylene glycol; propane-1,3-diol;butane-1,4-diol; polytetramethylene glycol; polyethylene glycol;polypropylene glycol; 1,4-hydroxymethylcyclohexane and the like,substituted for the glycol moiety in the preparation of the copolymerand/or isophthalic; bibenzoic; naphthalene 1,4- or 2,6-dicarboxylicadipic; sebacic; decane-1,10-dicarboxylic acid, and the like,substituted for the acid moiety in the preparation of the copolymer.

Preferably, the intrinsic viscosity of the polymeric material of thisinvention is between 0.51 to 0.85. Materials of this viscosity in flakeform may be obtained as start-up materials in the form of cut strands orbead trim or pelletized material or even pelletized flake. All of theseflakes are subsequently dried for a period of 2 hours at 80°C. with29-inch water under vacuum. It has been found that bulk densities of28-40 lbs./ft.³ handle well in single screw extrusion equipment.

The expression "intrinsic viscosity" is used herein as a measure of thedegree of polymerization of the polyester and is defined as: ##EQU2##wherein μr is the viscosity of a dilute solution of the polyester in amixture of 1 part trifluoroacetic acid and 3 parts methylene chloride,by volume, divided by the viscosity of the trifluoroaceticacid/methylene chloride mixture per se measured in the same units at25°C. and C is the concentration in grams of the polyester per 100 cc.of solution. Intrinsic viscosity is independent of concentration byvirtue of the extrapolation to C = 0.

Polyethylene terephthalate has an unusual number of properties whichenable it to serve a diverse range of applications. These balancedproperties enable it to be particularly usable as a packaging materialfor food. As an example, polyethylene terephthalate does not get brittlewith age, it has a long shelf life; it provides a permanent,nonyellowing, dimensionally stable base and has excellent resistance togrease, oxygen and moisture. Further, it is resistant to initial tearand scratching and can withstand heavy impact and prolonged flexing.These and other properties are of great value in packaging applications.

Preferably, the thickness of the paperboard is about 14 mils and thethickness of the polyethylene terephthalate layer is 1 mil orthereabouts.

In the practice of this invention, it has been found necessary to heatan uncoated first surface of the paperboard to a temperature of at least285°F. before the molten polyethylene terephthalate material is extrudedonto the paperboard. If the second surface of the paperboard is coated,a maximum temperature of 340°F. has been found possible. At temperaturesin excess of this when coating or extruding polyethylene terephthalateonto this uncoated second surface of the paperboard, it has been foundthat blocking occurs, primarily blocking of the coating on the firstsurface to the extruded material or coating on the second surfaceoccurs. Further, the coating tends to adhere to the chill roll at abovethis maximum.

A blank formed from the laminates, either 2-ply or 3-ply, of thisinvention, heat-seals easily at about 320°F. This property enables theblank to be folded, by folding the body parts along the fold linesupwardly with respect to the bottom part and the bringing of surfaceportions of the body parts together, such portions all being ofpolyethylene terephthalate or one being of paperboard and theheat-sealing of these portions together to form the container. Excellentheat seals of polyethylene terephthalate to itself, for example, may beobtained with densities of 1.3300 to 1.3360, as measured at 27°C. in aCCl₄ /heptane solid mixture.

Typical heat-sealing temperatures that have been found workable informing a container of this invention have included 320°F. at 3 secondsdwell and 60 psi and 355°F. at 3 seconds dwell and 60 psi, each of whichproduce a seal strength (fiber tearing) of greater than 3,000 grams/inchwhen a laminate of 14 mil-thick paperboard coated with polyethyleneterephthalate is heat-sealed with abutting surface portions of thepolyethylene terephthalate layer in contact with each other. When thislayer is heat-sealed to an uncoated side of the paperboard at 355°F. at6 seconds dwell and 60 psi a seal strength of about 600 grams/inch isobtained.

The containers of this invention are particularly well suited for bakingor cooking foods at temperatures up to about 400°F.

A typical method of cooking includes the steps of:

forming a container from a blank formed from a laminate including thenonbrowning paperboard having at least a layer of polyethyleneterephthalate extruded thereon and bonded thereto by its own substanceby folding the blank to bring surface portions of the polyethyleneterephthalate layer thereof into abutting contact with other surfaceportions of such layer and by heat-sealing these abutting surfaceportions together;

placing food in the container; and

cooking the food at a temperature of up to about 400°F.

Another method of cooking includes the steps of:

forming a container from a blank formed of a laminate including thenonbrowning paperboard having at least a layer of polyethyleneterephthalate extruded thereon and bonded thereto by its own substanceby folding the blank to bring surface portions of the polyethyleneterephthalate layer thereof into abutting contact with surface portionsof the paperboard layer and by heat-sealing these abutting surfaceportions together;

placing food in the container; and

cooking the food at a temperature of up to about 400°F.

Still another method of cooking includes the steps of:

forming a container from a blank formed from a laminate including thenonbrowning paperboard having at least a layer of polyethyleneterephthalate extruded thereon and bonded thereto by its own substance;placing food in the container; and cooking the food at a temperature ofup to about 400°F. This container may be formed by heating the blank toat least 320°F. and by pressing the heated blank or by folding theblank.

Lastly, another method of cooking includes the steps of:

forming a container from a blank formed from a laminate consisting ofthe nonbrowning paperboard having at least a layer of polyethyleneterephthalate extruded thereon and bonded thereto by its own substanceby folding the blank to bring surface portions of body parts thereofinto abutting contact with each other and by securing these abuttingsurface portions together; placing food in the container; and cookingthe food at a temperature of up to about 400°F.

In all of these cooking methods, the novel laminates of this inventionenable the cooking to be done under workable conditions and at unusuallyhigh temperatures, without paperboard browning problems, for example.

METHODS OF AND APPARATUS FOR MAKING THE LAMINATE

A preferred method of making the laminate of this invention includes thestep of extruding molten polyethylene terephthalate material onto anuncoated surface of paperboard material after such paperboard materialhas been flameheated to a temperature of at least 285°F.

Referring to FIG. 1, there is shown a schematic representation of anapparatus for making the material of this invention. Such material,whether 2-ply laminate 17 or a 3-ply laminate 17' is formed under noveland critical conditions.

In making such material, the paperboard 15, in the form of a web, forexample, is supplied from a supply roll 21 and is moved by appropriatemeans, not shown, past a heating means or source, such as flame or hotgas, 22 and into a nip formed by contacting rotating surfaces of a niproll 23 and a quench or chill roll 24 where the paperboard is combinedwith a layer of the freshly extruded molten polyethylene terephthalatematerial 16 to form the 2-ply laminate 17.

The molten material 16 is extruded onto the uncoated surface of thepaperboard 15 from an extrusion die 25. This molten material 16 descendsthrough an air gap until it contacts the first surface of the paperboard16 in the nip area. The paperboard and polyethylene terephthalate layersare firmly pressed together between the elastomeric covered roll 23 andthe chill roll 24 so that the molten polyethylene terephthalate layer 16is joined to or fused to the paperboard 15 by its own substance.

It is important that the pressure and heat applied are sufficient tocombine or fuse the paperboard and polyethylene terephthalate layertogether and it is critical that the first uncoated surface of thepaperboard be heated to a temperature of at least 285°F. prior to theextrusion of the polyethylene terephthalate layer onto such surface.

The laminate 17 is then passed by a second heating means 22', around anidler roll 26 and a nip roll 27 and into a nip formed by coactingsurfaces of the nip roll 27 and a quench roll 28. A second moltenpolyethylene terephthalate layer 16' is cast into the nip from anextrusion die 29 and onto the uncoated second surface of the heatedpaperboard 15 which is coated with the molten materials 16'. Theextruded molten polyethylene terephthalate layer 16' like the layer 16previously applied sets up strongly and is fused to the paperboardmember by its own substance to form the 3-ply laminate 17'.

After formation of the 3-ply laminate 17', it is moved onto a windup 30for further processing or use.

The second surface of the paperboard 15 is heated by the heating meansto a critical temperature of from at least 285°F. to about 340°F. Lowertemperatures than this negate desired bonding and higher temperaturesthan this tend to cause adherence of the coating to chill roll, forexample.

Specifically, the preferred method of making the 2-ply laminate 17 ofthis invention includes the steps of:

passing the nonbrowning paperboard web 15 having an uncoated surface anda pH of between 7.0 and 7.5 adjacent to a flame type heating means 22;

heating the uncoated first surface of the paperboard to a temperature offrom at least about 285°F. to about 340°F. by use of the flame;

extruding the layer of polyethylene terephthalate 16 onto the uncoatedsurface of the heated paperboard 15, such polyethylene terephthalatehaving an intrinsic viscosity within the range of 0.51 to 0.85; and

cooling the extruded layer of polyethylene terephthalate to solidify itand to effect a bond between the paperboard 15 and the polyethyleneterephthalate 16 to form the 2-ply laminate 17.

And, in making the 3-ply laminate of this invention wherein the secondsurface of the paperboard also is uncoated and wherein the paperboard isheated to a temperature of from about 285°F. to about 340°F., suchmethod includes the additional steps of:

extruding the layer of polyethylene terephthalate 16' onto such otheruncoated surface of the heated paperboard 15, such polyethyleneterephthalate having an intrinsic viscosity within the range of 0.51 to0.85; and

cooling this extruded layer of polyethylene terephthalate to solidify itand to effect a bond between it and the paperboard to form the 3-plylaminate 17'.

The preferred method of heating the paperboard surfaces is tocontiuously expose the surfaces of the moving paperboard to a flamesupplied by a gaseous mixture of propane and oxygen or propane and air.The flame which emerges from a suitable burner device is composed of aprimary envelope comprising the luminous, pale blue, conical innerportion which is adjacent to the burner orifice and the secondaryenvelope comprising the remainder of the flame. When a rectangularshaped burner orifice is employed the primary envelope is wedge-shapedin cross-section. The surface is passed through the primary envelope ofthe flame at a distance from the burner orifice sufficient formaintaining a self-sustaining flame. The gaseous constituents forcombustion are propane and oxygen or air. The proportions of theseconstituents that are supplied to the burner are not critical for thesuccessful flame heating of the paperboard surface.

The one and two-side polyethylene terephthalate coated paperboardlaminates of this invention may be used to construct a variety of foodcontainers such as trays, tray lids, roasting pans, and liquid-tightcontainers. The polyethylene terephthalate coating heat-seals well inaddition to having excellent barrier resistant properties to grease,water, and oxygen. The heat-seal property permits one to form leak-proofliquid packages and cooking trays and the polyethylene terephthalatecoating prevents the paperboard from excessive browning even at oventemperatures of up to about 400°F.

For the purpose of this invention, "container" means any plate, tray,folding carton or the like that is adapted to hold an article. It isfurther to be understood that all of these "containers " may be providedwith a top or lid which may be heat-sealed to an appropriate part orparts of the "container" wherein the lower surface of the top is of apolyethylene terephthalate material and is bonded to an inner or uppersurface or flange of the "containers" of this invention, in which theblanks from which they are formed are of a polyethyleneterephthalate/paperboard laminate of this invention. Typical"containers" which may be so formed are shown in U.S. Pat. No. 3,550,835to Persson, U.S. Pat. No. 3,257,786 to Harrison et al., and U.S. Pat.No. 3,178,017 to Bowman, for example.

I claim:
 1. A method of making a laminate comprising:passing anon-browning paperboard having a pH of between 7.0 and 7.5 adjacent aheating source; heating an uncoated first surface of such paperboard toa temperature of at least about 285°F. by means of the heating source;extruding a layer of polyethylene terephthalate onto such uncoated firstsurface of the heated paperboard, such polyethylene terephthalate havingan intrinsic viscosity within the range of 0.51 to 0.85; and cooling theextruded layer of polyethylene terephthalate to solidify it and toeffect a bond between the paperboard and the polyethylene terephthalateto form a 2-ply laminate.
 2. The method of claim 1 including theadditional steps of:passing the 2-ply laminate adjacent a heatingsource; heating the other uncoated second surface of such paperboard toa temperature of from about 285°F. to about 340°F. by use of the heatingsource; extruding a layer of polyethylene terephthalate onto suchuncoated second surface of the heated paperboard, such polyethyleneterephthalate having an intrinsic viscosity within the range of 0.51 to0.85; and cooling this extruded layer of polyethylene terephthalate tosolidify it and to effect a bond between it and the paperboard to form a3-ply laminate.
 3. The method of claim 1 wherein the uncoated firstsurface of such paperboard is heated to a temperature of at least about285°F. by means of a flame.